This invention relates to a telescopically extensible lifting column, in particular for the height adjustment of a camera, and consisting of an external column member resting on a base plate, a middle column member which is telescopically extensible from the external column member, and a central column member which is in turn extensible from the middle column member and on which the camera is mounted. There is also a drive device by which the middle column member can be pressurized pneumatically in relation to the stationary external column member by means of a piston/cylinder unit, with a traction cable or the like which is guided over at least one deflection member mounted rotatably on the middle column member and connected at one end to the central column member and at the other end to the external column member.
In a known column of the above type, the traction cable connection ensures that the central column member, on extension of the middle column member, moves at a higher speed then the middle member. By using the piston/cylinder unit as a drive member, the possibility is afforded of establishing a state of equilibrium, by greater or lesser air supply to the unit (by means of pumps for example,) in all extension positions of the column corresponding to required camera heights. The weight K1 of the camera, central column member etc. is compensated in this connection by the force Km which corresponds to the pressure Pm acting in the piston/cylinder unit. In the event of pneumatic pressurization of the piston/cylinder unit, the volume increase of this unit during the extension is of course taken into account.
To move the central column member together with camera from a lower position into a higher position with the force Km, a quantity of air is required which corresponds to the change in volume of the piston/cylinder unit taking place thereby, and to the pressure Pm corresponding to the force Km to be compensated.
It is already known to connect the cylinder chamber of the piston/cylinder unit to a relatively large storage chamber which is under the same pressure as the cylinder chamber. Corresponding to the greater volume made available, a proportionately great quantity of compressed air under pressure is necessary in order to lift the central column member and bring about a state of equilibrium. If the latter is achieved, then the central column member together with the camera etc. supported by it can, with only very low expenditure of effort, be retracted or displaced by hand into another height position. In the event of a volume ratio of the piston/cylinder chamber to the storage chamber of approximately 1:100, the pressure in the system hardly changes during such displacement, that is to say the change in pressure is so small that the central column is held in the position into which it has been transferred by hand by existing friction forces alone. If the lifting column is part of a stand, the stand feet in conventional embodiments are hollow and the chambers thus created are filled with compressed air at approximately 3 MPa. In general, this does not take place only when at the location where the lifting column is used but also at the location where the lifting column is stored. Filling of the storage chambers does not take place by means of pumps but with the aid of compressed air bottles, and it is even usual to fill up the storage chambers before transport to the place of use. The overall outlay for the production of such lifting columns is thus extraordinarily high and handling is awkward and complicated.
Also known is another solution to the problem of moving the column part carrying the camera to different height positions by hand. This takes place with the aid of a conically shaped expensive piston rod which interacts with a special seal which adapts to the different cross-section conditions during piston movement. Although in this case only low pressures are used, the possible displacement travel is likewise relatively limited, as is the life of the costly special seals.
Lastly, attempts have been made to incorporate spring accumulators with long spring excursion in the travel of the extensible column member, for example in the manner of clockwork springs.